The physics behind the spread of coughing and sneezing - it's time to abandon outdated models

From the first moment we heard about the corona virus, we knew how to recite the "2 meter rule" by heart. Although the infection mechanism is deciphered quite quickly, We did not have an exact model that explains the spread of shrapnel in an open or closed space. The World Health Organization and many of the world's countries are based on an outdated model from the XNUMXs that does not correspond to reality. In a study recently published in the prestigious journal for physicists, researchers from Europe reveal how the cough cloud spreads in a closed space under realistic conditions and the findings are surprising.

The droplets of saliva emitted from coughing or sneezing are the main cause of the infection of the corona virus. Although we all know how important it is to understand the mechanics behind the spread of droplets in closed and open spaces, we have not yet been able to describe a realistic model that will accurately demonstrate their rate of survival in the air and the effect of the environment on them. In recent years, the number of studies has increased significantly, especially with the beginning of the epidemic, but the difficulty of building a controlled experiment and collecting the data remains challenging. Until recently, Wells's model which was developed in the thirties is considered the classic and accepted model. The model is relatively simple - when we sneeze or cough, we emit droplets of different sizes. To simplify the complex picture, the model assumes that the droplets leave the body cavity without interaction between them. According to Wells, the smallest should not worry us at all, they will evaporate very quickly. In contrast, the larger ones will move ballistically and fall relatively quickly to the ground, with the heaviest ones moving up to a meter away. Based on this outdated model, the 2 meter law was established.

Unfortunately, there is no empirical data to support the model, if the opposite is true. In recent months, evidence has been accumulating that claims that keeping a distance of two meters really does not help, especially if the sneeze or cough is done in a closed space. In the last year, researcher Lydia Boroeva from MIT New York and her partners Shown in experiments that in closed spaces the 2 meter and 10 second rule does not apply. The cough cloud can survive for up to 8 minutes at a distance of up to XNUMX meters. The researchers explained the significant difference thanks to the moisture cloud that is thrown into the air and prevents the droplets from drying out easily. Prof. Emmanuel Villermaux from France explained before that the contradiction comes from the assumption that the law of the root of time is also valid in drops that are not isolated. The root law states that isolated droplets from the sneeze cloud environment will decrease in diameter at a rate determined by the root of time.

Surprisingly, despite the significant increase in the amount of research, the effect of the environment such as humidity or temperature on the survival of the droplets remains a mystery and is still a subject of debate. Beyond the experimental difficulty, those who approach computer models claim that simulating thousands of particles, processing the data and choosing realistic starting conditions are a very big challenge. For the first time, in an article published in the prestigious magazine Physical Review Letters, researchers from Europe were able to create a controlled and qualitative model of the movement of droplets in the air under different humidity conditions. In the model, the researchers demonstrated the survival of the droplets leaving at a temperature of 34 degrees Celsius with one hundred percent humidity, into a space at a temperature of 20 degrees and a humidity of between 50-90 percent. According to the researchers, these conditions are similar to closed rooms in many parts of the world.

The results were surprising - but at first at least. At an average humidity of 50 percent, particles larger than a hundred microns fall according to the Wells model, between 10 and 70 centimeters from the opening point. These are less harmful and comply with the guidelines of the World Health Organization. However, droplets on the order of ten microns behave in a completely different way. Their trajectory remains quite horizontal and they are driven away from the point of emission with the help of a vortex motion. The average droplets tend to stay in the environment of the moist cough cloud, thus significantly increasing their life time. In a room with 50 percent humidity, the droplets can survive 60 times longer than the time predicted by the Wells model, while at 90 percent, the droplets can survive 100 to 200 times longer than the outdated model. In contrast, the researchers showed that the root law actually accelerates for large particles. These particles tend to detach from the cloud, and with the help of air convection, the water vapor that evaporates from the drop moves away from the surface and increases the rate of evaporation.

The current study contradicts the the argument of the World Health Organization which assumes that particles larger than 5 microns move ballistically and demonstrates for the first time that their survival is much higher thanks to the particle cloud that surrounds them in a humid environment. The researchers aim to continue testing the model in different environmental conditions, but for the first time it can be clearly argued that the simplistic model of Wells on which everyone is based does not describe reality at all and we must be even more careful in closed spaces.

For the original article and additional references - click HERE

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